The mu-opioid receptor (MOR) is required for the profound effects of opiate drugs, including analgesia and drug reward. A long-term objective of the opioid research field is to understand the cellular and molecular targets through which the MOR mediates such effects. The MOR expresses broadly in the brain, so the precise cell types in which MOR expression is responsible for opiate's physiological effects including reward-related behaviors remain unknown. In the striatum, a major subcortical region implicated in drug addiction, MOR is expressed at high levels in a subset of medium spiny neurons (MSNs) that defines a striatal compartment, the striosome/patch compartment; while the MOR negative MSNs constitute the matrix compartment. The functional significance of striosome compartment and its high level MOR expression remain elusive. In our preliminary study, using a novel BAC transgenic rescue strategy, we have demonstrated that targeted expression of MOR selectively in the striosome MSNs that project to the substantia nigra (striatonigral MSNs) is sufficient to restore opiate reward seeking behaviors (i.e. sensitization and CPP) in otherwise MOR null background. Furthermore, the rescue mice also restore opiate-induced dopamine release in the striatum. In the current project, we will further define the striatal subregions (dorsal vs ventral), cell types (striatonigral vs striatopallidal MSNs in the striosome), and cell-autonomous gene expression changes that are critical to opiate reward. We will further test the mono-synaptic connectivity between the striosome striatonigral MSNs and the dopaminergic neurons in substantia nigra and ventral tegmental areas. Finally, we will use optogenetic and functional MRI approaches to test whether light-induced stimulation or suppression of the striosome striatonigral MSNs could lead to bi-directional modulation of reward-seeking behaviors including opiate reward, and to visualize the neural circuits controlled by striosome striatognigral neurons.